The roots of this asparagus fern have so compacted the soil that the soil holds no more water and the plant is constantly drought-stressed. Photo: ChaserGuy, Reddit
Over time, many houseplants and patio plants become pot-bound. The term “underpotted” is also used. Their roots have taken over the entire mass of potting soil, compressing it and leaving no room for further growth. Many flowering plants actually bloom more abundantly when slightly pot-bound, but when the roots are so numerous you can barely see the soil or circle round and round the inside of the pot, or they come crawling out of the pot through it’s drainage holes, you may need to do a bit of judicious root pruning.
You can quickly tell if a plant is pot-bound by turning it over and looking at the bottom of its pot. Roots growing out through the drainage hole are a sure sign it is pot-bound.
Of course, you could simply repot the pot-bound plant into a larger container, filling in all around the matted roots with a new mass of fresh soil and new roots will indeed grow into the growing mix offered, but eventually, especially in the case of indoor trees, enough is enough. You simply don’t want to keep potting them up into bigger and bigger pots: they’re big enough as it is. They’ll have to stay in the same size pot at some point. That’s when you’ll have to seriously root prune.
Removing a limited number of old roots doesn’t harm the plant. Instead, it usually gives it a new lease on live, stimulating massive new root growth. Just don’t overdo it. Pruning off no more than a third of its old roots at a time will be fine.
Natural Over Rooters
Some plants are more subject to becoming pot-bound than others.
Spider plants (Chlorophyum comosum) and asparagus ferns (Asparagus aethiopicus ‘Sprengeri’ [formerly A. densiflorus ‘Sprengeri’] and similar species) produce massive roots than seriously compact the soil in which they grow, reducing its water-holding capacity and leaving them constantly drought-stressed. You may need to root prune these annually.
This dracaena produced so many roots it actually pushed the plant’s root ball upward, lifting it part way out of its pot! Photo: http://www.ourhouseplants.com
Dracaenas (Dracaeana marginata and others) and similar indoor trees are given to producing long anchor roots that, having no where else to go, wrap around and around bottom part of the pot. They’re designed to hold the plant in place in the wild, but don’t do much in the way of water and mineral absorption. In a pot, they’re pretty much useless and can simply be removed.
For Everything, There is a Season
Ideally, you’ll want to repot when the plant is either growing or about to spring into growth: it will then recuperate faster. This can be any time from early spring to late summer, although it’s best to avoid repotting, especially when serious root pruning will be involved, when plants are in bloom. Late winter/earliest spring (late February and March in the Northern Hemisphere) is usually the best time for most houseplants.
Doing the Deed
Are you ready to unpot?
Well, first cut off any roots creeping out through the drainage holes into the saucer. The very fact that they’ve left the confines of the pot proves they’re pretty much useless and deserve to be cut right off. Plus, you’ll have a hard time removing the pot if stubborn roots are jamming the drainage holes and hanging on for dear life.
Once you’ve removed your plant from its pot and found it’s clearly pot-bound. What do you do?
Sometimes, especially when the offending roots are all on the bottom and curl around the pot, you can just pull them outwards, away from the root ball, and prune them off at the source with pruning shears.
If they’re everywhere, the treatment will need to be more draconian. Take a sharp knife and slice off about 2 to 3 cm (1/2 to 1 inch) of root ball all around the plant, including underneath. I actually use a pruning saw: yes, designed for pruning tree branches. It does a wonderful and fast job!
If you intend to repot into the same-size pot, for example an indoor tree you don’t want to see grow any more in height, cut off even more. As mentioned, you can prune off up to one third of the roots of most plants without doing them any harm.
That done, you might as well knock free some of the old potting soil, probably becoming contaminated with excess mineral salts anyway, loosening it gently with your fingers, a stake, a pencil or a chopstick.
Now, repot using fresh potting soil, centering the plant in its new home and working the new mix in and around the roots that are left. Then water well. Keep the plant in partial shade for a week or two, until it recuperates, then move it back to its original spot. Job done!